1. Technical Field of the Invention
This invention relates generally to electronic amplifiers, and more specifically to means for reducing switching glitches in amplifiers which switch between multiple power rails.
2. Background Art
Class AB amplifiers use two amplifier devices in a push-pull configuration, with both amplifier devices biased slightly on, to avoid distortion glitches at the crossover. The amplifier is driven with a fixed, predetermined pair of power rails, typically VDD and GND. Class AB amplifiers are not especially efficient, and waste considerable power due to significant quiescent currents.
Class G amplifiers are, essentially, an improved and more efficient version of Class AB amplifiers, designed to reduce this inefficiency. Class G amplifiers are provided with more than two power rails, and are switched between power rails depending upon the input signal, gain setting, and so forth. The general principle of Class G is to use the lowest power rails that will provide adequate power at the particular instant. A Class G amplifier draws power from low voltage supply rails for generating small output signals, and only uses high voltage supply rails for the peak output signal excursions.
A known problem with Class G amplifiers is that a glitch occurs at the “crossover” or “handoff” point where the output signal current switches from an inner (low voltage) to an outer (high voltage) power supply rail or vice versa. These glitches cause increased distortion and possible EMI related problems. The glitches are caused by inevitable mismatches in the gains between the two signal paths and the finite, non-zero time required to divert the source of the output signal current from one set of power supplies to another. The switching is typically done very abruptly, to minimize the energy of the glitch. However, this abruptness causes the frequency of the glitch components to be outside the bandwidth of the amplifier and thus not susceptible to being reduced by the negative feedback mechanisms used to linearize other aspects of the amplifier.
FIG. 1 illustrates one example of a conventional Class G amplifier 10. The amplifier is provided with two pairs of power rails: a high voltage pair VDD to VSS, and a low voltage pair VDDL to VSSL. Voltage supplies and bias generators are well-known in the art, and thus are omitted from FIG. 1 for the sake of clarity in the illustration.
The amplifier includes a positive side outer current mirror CM1 coupled to the high voltage positive power rail VDD, a positive side inner current mirror CM2 coupled to the low voltage positive power rail VDDL, a (relatively) negative side inner current mirror CM3 coupled to the low voltage negative power rail VSSL, and a (relatively) negative side outer current mirror CM4 coupled to the high voltage negative power rail VSS.
A transconductance amplifier (gm Amp) is coupled to receive an input signal VIP; often, this is one half of a VIP/VIN differential input pair, one of which is tied to an analog ground point. A current sink output of the transconductance amplifier is fed to the input of a positive side switching device 12. The two outputs of the positive side switching device are respectively coupled to the inputs of the positive side current mirrors. The outputs of the positive side inner and outer current mirrors are both coupled to the amplifier's output terminal VOUT, and the output of the positive side outer current mirror is also coupled to a positive input of a positive side comparator 14. The negative input of the positive side comparator is coupled to a threshold voltage reference that tracks the positive side low voltage power rail. The threshold voltage is set to a value that just prevents the inner rail output stage from saturating before passing control to the outer rail output stage, and is typically around 200 mV. The output of the positive side comparator is coupled to control the positive side switching device.
The negative side or half of the amplifier is a mirror image of the positive side, using a negative side switching device 16 and a negative side comparator 18.
FIG. 2 illustrates the output current provided by the positive side lower supply (inner current mirror) in response to a particular input signal. As the input signal rises, the inner current mirror's output current rises accordingly until, at a particular point 20 the inner current mirror's power rails are inadequate, and the inner current mirror is switched off.
FIG. 3 illustrates the output current provided by the positive side upper supply (outer current mirror) in response to that same input signal. As long as the inner current mirror's power rails are adequate, the outer current mirror is not utilized until at the point 20, when the positive side comparator triggers the positive side switch to turn the positive side inner current mirror off and the positive side outer current mirror on. Then, the positive side outer current mirror provides the current for the output signal until, at a particular point 22 the input signal falls enough that the inner current mirror's power rails are again adequate. At that point, the positive side comparator triggers the positive side switch to turn the positive side outer current mirror off and the positive side inner current mirror on, as shown in FIG. 2.
FIG. 4 illustrates the composite output current provided by the combination of the positive side inner and outer current mirrors in response to that input signal. The “on” portions of the two waveforms of FIGS. 2 and 3 would, ideally, form a smooth, glitch-free waveform when combined. However, due to problems inherent in Class G amplifiers, switching glitches do appear in the output current waveform at the points 20, 22 where the amplifier switches between current mirrors.
FIG. 5 illustrates a waveform demonstrating the output 24 of a positive side inner supply and the output 26 of a positive side outer supply, particularly showing them crossing over at a point 28 when the inner supply's power rails have just become inadequate.
FIG. 6 illustrates a waveform of the composite output 30 provided by the positive side inner and outer supplies, zoomed in to particularly show the large glitch 32 produced at the crossover (at point 28 in FIG. 5).
What is needed, then, is an improved Class G amplifier and method of operating it, which reduces, minimizes, or eliminates such switching glitches.